1. Field of the Invention
The present invention relates to novel imido polymers, and, more especially, to novel imido copolymers comprising hindered diamine recurring units which have good mechanical and electrical properties and which are particularly heat-stable at elevated temperatures.
2. Description of the Prior Art
French Patent Application FR-A-2,608,613 describes imido polymers, including heat-curable prepolymers, which comprise the product of reaction, at a temperature ranging from 50.degree. C. to 300.degree. C., between:
(a) an N,N'-bisimide or a plurality of bisimides of the formula: ##STR1## in which each of the symbols Y, which may be identical or different, is H, CH.sub.3 or Cl; and the symbol A is a divalent radical selected from among the following: cyclohexylene, phenylenes, 4-methyl-1,3-phenylene, 2-methyl-1,3-phenylene, 5-methyl-1,3-phenylene and 2,5-diethyl-3-methyl-1,4-phenylene, and the radicals of the formula: ##STR2## wherein T is a single valence bond or one of the groups: ##STR3## and each of the symbols X, which may be identical or different, is a hydrogen atom or a methyl, ethyl or isopropyl radical;
(b) one or more hindered diprimary diamine(s) selected from among:
(i) the species corresponding to the general formula: ##STR4## in which each of the symbols R.sub.1, R.sub.2, R.sub.3 and R.sub.4, which may be identical or different, is a methyl, ethyl, propyl or isopropyl radical; and each of the symbols Z, which also may be identical or different, is a hydrogen atom or a chlorine atom; and (ii) the species corresponding to the general formula: ##STR5## in which the amino radicals are in a meta or para position relative to each other; and each of the symbols R.sub.5, which may be identical or different, is a methyl, ethyl, propyl or isopropyl radical; PA0 (f1) a chlorinated or brominated epoxy resin; PA0 (f2) an N,N'-alkylenebistetrahalophthalimide of the formula: ##STR10## in which each of the symbols U, which may be identical or different, is a chlorine or bromine atom; the alkylene radical --C.sub.n H.sub.2 n-- may be linear or branched; and n is an integer equal to 1, 2, 3 or 4; PA0 (f3) a halogenated compound of the formula: ##STR11## in which the symbols U are as defined above; the symbol V is a single valence bond, a linear or branched chain alkylene radical --C.sub.n H.sub.2n --, with n being equal to 1, 2, 3 or 4, or an oxygen atom; and p is an integer equal to 2, 3 or 4; PA0 (f4) an unhalogenated epoxy resin; and PA0 (f5) a mixture of two or more of the above-mentioned compounds (f1) to (f4) .
(c) optionally, one or more unhalogenated monomer(s) other than a bisimide of formula (I) and comprising one or more polymerizable carbon-carbon double bond(s); and
(d) optionally, an imidazole compound.
One advantage of such imido polymers is provided by the steric hindrance of the amine reactant (b), which is responsible for a lower reactivity of the constituents of the polymerization mixture when compared with the polyaminobismaleimides prepared from unhindered diamines. This lower reactivity of the constituents of the polymerization mixture is of special interest not only for the production of articles requiring a prepolymer in the molten state, but also for the production of articles requiring a prepolymer in the form of solution in a solvent. Indeed, the viscosity of the prepolymer in the molten state, or that of the solution of the prepolymer, changes more slightly and this greatly facilitates the processing of the prepolymer, especially in applications requiring a working life of a number of hours.
A second advantage of the imido polymers described in the '613 published French application is that such polymers are prepared without requiring special precautions from the standpoint of health and safety, given that the hindered diamines (b) employed do not present the toxicity hazards which are generally associated with unhindered aromatic diprimary amines.